docs: help pages for export, sessions and the status bar

src/help/13-sessions.txt

@@ -0,0 +1,34 @@
+<b>Sessions and settings</b>
+
+udpscope stores one session file per source, so reopening a source gives you the scope you
+closed. There is no global preferences file: the session <i>is</i> the settings.
+
+<b>How to reach it</b>
+
+Nothing to open — the file is written as you work. Its path is in the About dialog, and the
+directory is <tt>$XDG_CONFIG_HOME/udpscope</tt>, or <tt>~/.config/udpscope</tt> when that is
+unset. Each file is named <tt>&lt;host&gt;_&lt;port&gt;.toml</tt>.
+
+<b>Worth knowing</b>
+
+• What a session persists: the time window, the rows and each row's slot signals, the
+  calibration, which signals are enabled, the math channels with their graphs, and the
+  measurements pane.
+• Signal identity is the <i>name</i> throughout, because a new CONFIG renumbers the channels.
+• A file written by a run against a source whose streamer was down would name no signals at all,
+  so such a run does <b>not</b> write: a configured session is never overwritten by a run that
+  configured nothing.
+• The multicast group and the interface live <i>inside</i> the file, because its name cannot
+  carry them. A file from an older build has neither key and loads as unicast on the default
+  route.
+• The startup chooser lists only files that name at least one signal, which is what keeps it the
+  sources you configured rather than every port you have ever tested against.
+• Deleting a file is a supported way to start a source over; the chooser's <tt>×</tt> does
+  exactly that.
+
+<b>Example</b>
+
+Connect to <tt>127.0.0.1:8000</tt>, drop three signals on row 1 and set a 2 s window.
+<tt>~/.config/udpscope/127.0.0.1_8000.toml</tt> now holds <tt>rows=1 window_s=2</tt> with
+<tt>signals = ["sine", "ramp", "noise", ""]</tt>, and the next run reports
+<tt>layout loaded from …/127.0.0.1_8000.toml</tt> with those signals already acquired.